The difference between single and double screw compressor chiller unit
1. The influence of load change on energy efficiency ratio
The multi-head chiller can reduce the operation of some compressors when the cooling system load is reduced to a certain extent (multi-head is a modular combination of multiple screw compressors, when the water temperature drops close to the set range, it will The proportional integral calculation of the outlet water temperature will close the compressor with long running time and many starts in the module combination), which is also an important reason why users like it.
In this way, for a compressor, it is operated under the load of 60% to 100% of the air conditioning system, so as to achieve the purpose of energy saving. It is important to note that the discussion here is of the efficiency of one (or more) of the compressors, not of the chiller. As a rule, the optimum efficiency of the compressor and the motor should be about 50% to 90%. Efficiency drops sharply when the compressor and motor operate below 50%. The load variation of the single-head chiller is large and the adjustment range is wide. According to the change of the actual load, the single-head chiller can change accordingly, which can save part of the power consumption.
When the cooling load is between 60% and 80%, the condenser and evaporator of the unit increase the heat exchange margin, that is, the relative heat exchange area of the condenser and the evaporator is increased, and the relative heat dissipation area of the cooling tower is also increased. The heat exchange temperature difference between refrigerant and chilled water, air and cooling water is reduced, the condensing temperature of the unit is lowered, and the evaporating temperature is raised, thereby improving the efficiency of the unit and reducing the user's cost of use. When the cooling load of the unit is 60%, the most power is saved, the power consumption per unit cooling ton is 0.6kW/ton, and the energy efficiency ratio of the unit is 5.80 at this time. For multi-head chillers, since each refrigeration system is usually independent, it is difficult for each compressor to utilize the condenser and evaporator area of the entire unit.
For example, for a unit with two compressor heads, when the load of the entire air conditioning system drops to 50%, one compressor is turned off, and only one of the compressors is allowed to operate. At this time, the running compressor cannot be used. Close the side of the condenser and evaporator area. A running compressor is actually still running at full load, which will result in a drop in the part-load efficiency of the multi-head unit.
2. Unit reliability
Whether the user can use the air conditioning system normally, the reliability of the unit is very important. So how is the reliability of the unit calculated?
When the unit uses multiple compressors, the number of moving parts of the unit is doubled or more. The more moving parts a unit has, the less reliable the unit is. Because under the same technical level, the failure rate of the unit is proportional to the number of components and the number of control objects in the control system. The single-head unit is equipped with a set of refrigeration system and a single control element, with few control objects and high control accuracy, so the reliability of the unit is higher and the service life is longer. In the multi-head unit of the frequency conversion chiller, multiple compressors are configured, and one controller needs to control multiple control components. Due to the complex system and various equipment, the reliability of the unit is relatively low and the service life is relatively short. Based on this situation, some manufacturers have improved the design so that each machine head can operate relatively independently, reducing the impact of a single system failure on the whole machine.
3. Load regulation
For single-head screw refrigeration units with large cooling capacity, manufacturers generally use well-designed sliders to achieve stepless adjustment within the cooling capacity range of 15% to 100%, which not only provides users with a comfortable air-conditioning environment, but also And save energy. In the load regulation of the refrigerator, the high-efficiency operation of the refrigerator can be maintained for different working conditions. In the multi-head unit, the cooling capacity is adjusted by stages, and the cooling capacity is generally adjusted by starting and stopping the compressor, so the load adjustment effect is poor. At the same time, when the load of the multi-head unit is adjusted, it is often operated by starting and stopping the compressor, which on the one hand affects the accuracy of the adjustment; The service life of the unit will have a great negative impact.
4. Influence of starting current on other electrical equipment
Under certain air-conditioning load conditions, the multi-head unit can start multiple heads one after another when starting, so that the smaller head has a smaller starting current, so the multi-head unit will have a good performance in this regard. Performance.
5. On-site installation and maintenance
Under the condition of the same cooling capacity, the volume of the single-head screw unit is relatively smaller, so the installation is flexible and convenient. Due to the few cooling and chilled water interfaces, the single-head unit requires less engineering than multi-head units during on-site installation; the single-head unit has high operational reliability, so maintenance is simple. In addition, in the multi-head unit, there are many refrigeration pipeline interfaces, which leads to an increase in the leakage points of the system and increases the maintenance amount of daily operation. When the single-head chiller cannot meet the cooling capacity, the chiller manufacturer will recommend the use of a double-head or multi-head chiller.
Moreover, the energy of the chiller can also be adjusted, but when adjusting the chiller, individual attention should be paid:
1) Chiller with only 1 compressor: During the heating process, the compressor will automatically start when the displayed temperature ≥ the set temperature + temperature difference; during the cooling process, the compressor will automatically stop when the displayed temperature < set temperature + temperature difference .
2) Chiller with 2 compressors: During the heating process, when the displayed temperature > set temperature, 1 compressor will be automatically started, and when the displayed temperature ≥ set temperature + temperature difference, 2 compressors will be automatically started, and when the temperature is lowered, 2 compressors will be automatically started. During the process, when the display temperature < set temperature, one will automatically stop, and when the display temperature < set temperature - temperature difference, it will stop all.
